Security improvements:
- Restrict cert fetches to a host+port allowlist (Microsoft and DigiCert
on 80/443).
- Route requests through a dedicated `http.Client` that resolves the
host once and dials the validated IP directly, preventing DNS rebinding.
- Reject loopback, private (RFC 1918 / IPv6 ULA), link-local, multicast,
unspecified, CGNAT, benchmarking, and IPv4-mapped IPv6 addresses.
- Cap the certificate response body at 1 MiB.
- Log the underlying error via slog and return a generic detail to the
caller to prevent information disclosure.

(cherry picked from commit 57b11d4)